1. Field of the Invention
This invention relates to solid rocket propellant formulations. More specifically, the present invention relates composite propellant formulations which may be mixed, cast, and cured at ambient temperature.
2. Technology Background
Solid propellants are used extensively in the aerospace industry and are a preferred method of powering most missiles and rockets for military, commercial, and space applications. Solid rocket motor propellants have become widely accepted because they are relatively simple to manufacture and use, and because they have excellent performance characteristics.
Typical solid rocket motor propellants are formulated using an oxidizing agent, a fuel, and a binder. At times, the binder and the fuel may be the same. In addition to the basic components, it is conventional to add various bonding agents, plasticizers, curing agents, cure catalysts, and other similar materials which aid in the processing or curing of the propellant or contribute to mechanical properties of the cured propellant. A significant body of technology has developed regarding the processing and curing of solid propellants.
Many types of propellants used in the industry incorporate ammonium perchlorate (AP) as the oxidizer. The AP is generally incorporated into the propellant in particulate form. In order to hold the propellant in a coherent form, the components of the propellant are bound together by a polymeric binder, such as, but not limited to, a hydroxy-terminated polybutadiene (HTPB) binder. Such binders are widely used and commercially available. It has been found that such propellant compositions are easy to manufacture and handle, have good performance characteristics, and are economical and reliable. As a result, this type of solid propellant has become a standard in the industry.
Propellants must generally meet various mechanical and chemical performance criteria to be considered acceptable for routine use. For example, it is important that the propellant have desired mechanical characteristics which allow it to be used in a corresponding rocket or missile. It is important that the propellant deform elastically during use to avoid cracking within the propellant grain.
If the propellant cracks, burning within the crack may be experienced during operation of the rocket or missile. Such burning in a confined area may result in an increased surface area of burning propellant or increased burn rate at a particular location. This increase in the burn rate and surface area can directly result in failure of the rocket motor due to over pressurization or burn through of the casing.
Accordingly, propellants are typically subjected to standardized stress and strain tests. The typical configuration of the propellant sample tested is often referred to as a JANNAF Class C specimen. The shape and size of such specimens are standard in the industry. Such specimens are typically placed in an Instron.RTM. testing apparatus and then pulled until the specimen fails. Data is recorded during such tests and objective measures of stress and strain performance are provided.
To make certain that propellant formulations meet the applicable specifications, it is often necessary to employ a bonding agent within the propellant composition. Bonding agents are widely used throughout the solid propellant industry to strengthen the polymeric matrix which binds the oxidizer and fuel together. They help to incorporate solid oxidizer particles into the polymeric binder system. Use of a bonding agent typically improves the stress and strain characteristics of the propellant.
A number of bonding agents are known and conventional. One class of bonding agents are the polyamine bonding agents TEPANOL.RTM. (tetraethylenepentamine acrylonitrile glycidol adduct) and TEPAN.RTM. (partially cyanoacrylated tetraethylenepentamine). TEPANOL.RTM. and TEPAN.RTM. are useful as bonding agents and improve the mechanical properties of isocyanate cured HTPB propellants. TEPANOL.RTM. and TEPAN.RTM. are believed to become chemically linked to the polymeric propellant binder. TEPANOL.RTM. and TEPAN.RTM. also electrostatically coordinate with the AP after forming a perchlorate salt from an acid/base reaction with AP. Thus, TEPANOL.RTM. and TEPAN.RTM. aid in binding the AP particles into the propellant matrix.
TEPANOL.RTM. and TEPAN.RTM., however, also cause difficulty in the formulation of propellant. TEPANOL.RTM. and TEPAN.RTM. are relatively basic, and in the presence of AP they produce a significant amount of ammonia. This makes it necessary to conduct propellant mixing steps under vacuum and to mix for long periods of time in order to substantially remove the produced ammonia. It often requires 24 hours or more to adequately remove the ammonia from TEPANOL.RTM. and TEPAN.RTM. systems. This significantly extends propellant processing time and increases costs. Insufficient removal of the ammonia can result in soft cures and nonreproducible mechanical properties because the free ammonia reacts with some of the isocyanate curing agent. These characteristics of TEPANOL.RTM. and TEPAN.RTM. result in significant disadvantages, such as long mix time, high labor costs, and AP attrition.
In another important class of bonding agents, the aziridines (i.e., cyclic ethylene imines), it is believed that a polymeric shell is formed directly around the oxidizer particles by homopolymerization, catalyzed by acidic AP. This hydrophobic layer is then more compatible with the continuous binder phase and results in better bonding of the AP particles. Since this reaction does not occur on nitramine surfaces, aziridines are limited to AP propellants.
Isophthaloyl-bis(methyl-ethyleneimide), known as HX-752 in the industry, is a widely used aziridine bonding agent. HX-752 has the following chemical structure: ##STR1##
HX-752 is believed to be incorporated into the propellant matrix by ring opening polymerization. HX-752 avoids the production of large amounts of ammonia which plague processes using TEPANOL.RTM. and TEPAN.RTM.. As a result, some advantages are derived from the use of HX-752.
Even in view of the foregoing, HX-752 is far from ideal as a bonding agent. One significant problem is that of economics. HX-752 presently costs from four to five times as much as TEPANOL.RTM.. Also, propellants produced using HX-752 have a relatively high mix viscosity, which inhibits processing. HX-752, as used in the industry, does produce some ammonia which may require extra vacuum mixing. Finally, it is also believed that HX-752 may be a carcinogen. Thus, it can be seen that the cost and chemical characteristics of HX-752 make it a less than ideal bonding agent.
Another disadvantage of TEPANOL.RTM., TEPAN.RTM., HX-752 and other known bonding agents is that they require processing and curing at elevated temperatures, about 135.degree. F. The elevated curing temperature speeds the curing process. At ambient temperatures, about 80.degree. F., cure times are often six to eight weeks, which is unacceptably long. While cure catalysts can be used to shorten cure times, doing so often results in inadequate potlife and inferior mechanical properties when compared to the same propellant processed and cured at higher temperature. Lower end-of-mix (EOM) viscosity and longer potlife may be obtained by increasing the level of plasticizer in the formulation (at the expense of mechanical properties), or finding replacements for ingredients which are detrimental to processing.
Accordingly, it would be an advancement in that art to provide composite propellant formulations containing bonding agents which may be mixed, processed, and cured at ambient temperature without raising propellant viscosities and without producing significant quantities of ammonia. Such propellant formulations would contribute to lower power requirements, shorter mixing times, lower labor costs, faster mixer turnaround times, and less AP attrition. It would also be an advancement in the art to provide such ambient processed and cured propellants having acceptable stress and strain characteristics.
Such composite propellant formulations are disclosed and claimed herein.